The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reverse osmosis systems are used to provide fresh water from brackish or sea water. A membrane is used that restricts the flow of dissolved solids therethrough.
A reverse osmosis system involves pressurizing a solution with an applied pressure greater than an osmotic pressure created by the dissolve salts within the solution. The osmotic pressure is generally proportional to the concentration level of the salt. The approximate osmotic pressure in pounds-per-square-inch is the ratio of the salt mass to water mass times 14,000. A one-percent solution of salt would have an osmotic pressure of about 140 psi. Ocean water typically has a 3.5 percent concentration and an osmotic pressure of 490 psi.
Water extracted from a reverse osmosis system is called permeate. As a given body of saline solution is processed by the reverse osmosis membrane, the concentration of the solution is increased. At some point, it is no longer practical to recover permeate from the solution. The rejected material is called brine or the reject. Typically, about 50% of recovery of permeate from the original volume of sea water solution reaches the practical limit.
Referring now to FIG. 1, a reverse osmosis system 10 is illustrated having a membrane array 12 that generates a permeate stream 14 and a brine stream 16 from a feed stream 18. The feed stream 18 typically includes brackish or sea water. A feed pump 20 coupled to a motor 22 pressurizes the feed stream 18 to the required pressure flow which enters the membrane array 12.
The permeate stream 14 is purified fluid flow at a low pressure. The brine stream 16 is a higher pressure stream that contains dissolved materials blocked by the membrane. The pressure of the brine stream 16 is only slightly lower than the feed stream 18. The membrane array 12 requires an exact flow rate for optimal operation. A brine throttle valve 24 may be used to regulate the flow through the membrane array 12. Changes take place due to water temperature, salinity, as well as membrane characteristics, such as fowling. The membrane array 12 may also be operated at off-design conditions on an emergency basis. The feed pumping system is required to meet variable flow and pressure requirements.
In general, a higher feed pressure increases permeate production and, conversely, a reduced feed pressure reduces permeate production. The membrane array 12 is required to maintain a specific recovery which is the ratio of the permeate flow to feed flow. The feed flow or brine flow likewise requires regulation.
A pretreatment system 21 may also be provided to pre-treat the fluid into the membrane array 12. The pretreatment system 21 may be used to remove solid materials such as sand, grit and suspended materials. Each of the embodiments below including those in the detailed disclosure may include a pretreatment system 21.
Referring now to FIG. 2, a system similar to that in FIG. 1 is illustrated with the addition of a feed throttle valve 30. Medium and large reverse osmosis plants typically include centrifugal-type pumps 20. The pumps have a relatively low cost and good efficiency, but they may generate a fixed pressure differential at a given flow rate and speed of rotation. To change the pressure/flow characteristic, the rate of pump rotation must be changed. One way prior systems were designed was to size the feed pump 20 to generate the highest possible membrane pressure and then use the throttle valve 30 to reduce the excess pressure to meet the membrane pressure requirement. Such a system has a low capital cost advantage but sacrifices energy efficiency since the feed pump generates more pressure and uses more power than is required for a typical operation.
Referring now to FIG. 3, another system for solving the pressure/flow characteristics is to add a variable frequency drive 36 to operate the motor 22 which, in turn, controls the operation of the feed pump 20. Thus, the feed pump 20 is operated at variable speed to match the membrane pressure requirement. The variable frequency drives 36 are expensive with large capacities and consume about three percent of the power that would otherwise have gone to the pump motor.
Referring now to FIG. 4, a system similar to that illustrated in FIG. 1 is illustrated using the same reference numerals. In this embodiment, a hydraulic pressure booster 40 having a pump portion 42 and a turbine portion 44 is used to recover energy from the brine stream 16. The pump portion 42 and the turbine portion 44 are coupled together with a common shaft 46. High pressure from the brine stream passes through the turbine portion 44 which causes the shaft 46 to rotate and drive the pump portion 42. The pump portion 42 raises the feed pressure in the feed stream 18. This increases the energy efficiency of the system. The booster 40 generates a portion of the feed pressure requirement for the membrane array 12 and, thus, the feed pump 20 and motor 22 may be reduced in size since a reduced amount of pressure is required by them.
Referring now to FIG. 5, a membrane element 60 that is suitable for positioning within a membrane array 12 of one of the previous Figs. is illustrated. The element 60 includes leaves of membrane material wrapped into a spiral configuration and placed in a thin tube 62 of material such as fiberglass. Each membrane leaf includes two membrane sheets glued on three sides with the fourth side attached to a central permeate pipe 64. Spacer grids (not shown) keep the membrane sheet from collapsing under the applied pressure. Feed solution enters one end of the membrane array 60 in the direction indicated by arrows 66. The solution or feed flows axially along the membrane element 60 and between the leaves 68 and exits through the high pressure brine outlet as indicated by arrows 70. Permeate is collected from the leaves 68 through permeate pipe 64. The pressure of the permeate through the tube 64 is essentially zero since the applied pressure is used to overcome the osmotic pressure and frictional losses of the flow of feed material through the membrane is performed.
Referring now to FIG. 6, a pressure vessel 78 that includes a plurality of membrane elements referred to collectively with reference numeral 60 is illustrated. In this example, three membrane elements are disposed within the pressure vessel 78. Each is denoted by a numerical and alphabetical identifier. In this example, three membrane elements 60a, 60b and 60c are provided in the pressure vessel 78. The pressure vessel 78 includes a first end cap 80 at the input end and a second end cap 82 at the outlet end. Feed is introduced into the pressure vessel in the direction of the arrows 84.
In this example, the three membrane elements 60a-60c are placed in series. Each subsequent element extracts a smaller amount of permeate than the preceding element due to an increasing osmotic pressure and decreasing applied pressure caused by frictional losses within the membrane elements. As a consequence, the final element 60c may produce very little permeate. The permeate pipe 64 collects permeate from each of the membrane elements 60a-60c. 
A typical reverse osmosis system operates at a constant pressure that is developed at the feed pump 20. The result is that an excess of applied pressure at the first membrane array may result in an undesirably high rate of permeate extraction which may allow the membranes to be damaged. The final membrane element 60c may have an undesirably low rate of extraction which may result in permeate with an excessive amount of salt contamination.